BEIRUT -- International inspectors on Tuesday entered the Syrian town where an alleged chemical attack was carried out earlier this month, following delays by Syrian and Russian authorities.

The fact-finding mission from the Organization for the Prohibition of Chemical Weapons is investigating reports that government forces launched an April 7 chemical attack in the final stages of their campaign to retake the town from rebels. The alleged gas attack, which Syrian activists say killed more than 40 people, prompted punitive U.S., British and French airstrikes.

Syria and its ally Russia deny any chemical attack took place, and Russian officials have accused Britain of staging a "fake" chemical attack. British Prime Minister Theresa May says Syria and Russia -- whose forces now control the town east of Damascus -- are trying to cover up evidence.

Journalists were allowed access to the suspected attack sites on Monday, but the OPCW said Syrian and Russian authorities blocked the inspectors.

The Associated Press spoke to survivors and witnesses who described being hit by gas. Several said a strange smell started spreading and people screamed, "It's chlorine! It's chlorine!"

The U.S. and France say they have evidence that Syrian President Bashar Assad's forces used poison gas in the attack, but they have not provided any evidence, even after Saturday's punitive missile strikes.

Douma was the last rebel-held town near Damascus, and the target of a government offensive in February and March that killed hundreds of people and displaced tens of thousands. Hours after the alleged chemical attack, the rebel faction that controlled the town, the Army of Islam, relented and was evacuated along with thousands of residents.

The site visit came hours after reports of more international air strikes on Syrian military installations. The Syrian military later said a false alarm set off air defence systems early Tuesday, retracting earlier reports of a pre-dawn "outside aggression" on its airfields in the central Homs region and a suburb of Damascus.

The Pentagon denied any American military activity in the area. There was no comment from Israel, which frequently carries out airstrikes in Syria but rarely acknowledges them.

Explosions were heard in the areas of the two bases, according to the Britain-based Syrian Observatory for Human Rights, which monitors the conflict through a network of sources inside the country. But it said no missiles landed inside the bases.

Earlier this month, four Iranian military personnel were killed in an airstrike on Syria's T4 air base, also in Homs. Syria and its main allies Iran and Russia blamed Israel for that attack. Israel did not confirm or deny the allegations.

The lack of access to Douma has left unanswered questions about the attack earlier this month. OPCW Director-General Ahmet Uzumcu said Syrian and Russian officials cited "pending security issues" in keeping its inspectors from reaching Douma.

"The team has not yet deployed to Douma," Uzumcu told an executive council meeting of the OPCW in The Hague on Monday.

Instead, Syrian authorities offered them 22 people to interview as witnesses, he said, adding that he hoped "all necessary arrangements will be made ... to allow the team to deploy to Douma as soon as possible."

Earlier Monday, Russian Deputy Foreign Minister Sergei Ryabkov said the inspectors could not go to the site because they needed approval from the U.N. Department for Safety and Security. He denied that Russia was hampering the mission and suggested the approval was held up because of the Western airstrikes.

However, U.N. spokesman Stephane Dujarric said the United Nations had provided all the necessary clearances for the team to visit Douma.

On Monday, the AP visited a two-room underground shelter where Khaled Mahmoud Nuseir said 47 people were killed, including his pregnant wife and two daughters, 18-month-old Qamar and 2 1/2-year-old Nour. A strange smell lingered, nine days after the attack.

Nuseir, 25, said he ran from the shelter to a nearby clinic and fainted. After he was revived, he returned to the shelter and found his wife and daughters dead, with foam coming from their mouths.

He and two other residents accused the rebel Army of Islam of carrying out the attack. As they spoke, government troops were nearby but out of earshot. Nuseir said a cylinder was found leaking the poison gas, adding that he didn't think it was dropped from the air because it still looked intact.

Separately, the AP spoke to a medic who was among those who later were evacuated to northern Syria. Ahmed Abed al-Nafaa said helicopters were flying before the attack and when he reached the site, people were screaming "chlorine." He said he tried to enter the shelter but was overcome by a strong smell of chlorine and his comrades pulled him out.